This invention relates generally to synchronous time division multiplexing (TDM) of signals and data and, more particularly, to a TDM system in which a particular signal can be sampled more than once in a single frame or when synchronous data is to be transmitted at multiples of some basic rate.
In the conventional TDM communication system a plurality of different signals is sampled periodically in sequence typically in a voice communication system at a sampling rate of 8000 periods/sec. The sampling period or frame is, therefore, 125 .mu.sec long and is subdivided into a plurality of equal duration time slots or channels. Each slot is dedicated to a specific one of the sampled signals except typically for certain slots which may be used for signalling and synchronization purposes. Each sample typically is a pulse code modulated (PCM) value represented by 8 bits. Such transmission systems exist in which the number of slots per frame is 1024 and higher.
To date, in all of the conventional TDM systems, a particular signal is sampled only once in each frame. It has been recognized that a need exists for transmitting signals or data of higher bit rates in the same network serving basic rate data. However, multi-slot (multi-channel) switching gives rise to difficulties such as the preservation of time slot order and frame consistency.
More particularly, at source encoding, a wide-band signal will naturally be sampled at equispaced instants in the frame period (125 .mu.sec in telephony systems) and at termination the samples must be delivered to the receiver in the same order and they must belong to the same frame. The network comprises a number of switching nodes interconnected by trunk groups. A switching node, in turn, may comprise a single-stage time switch or arrays of time switches interconnected either by links or space switches. At each time switch, a sample is written during a time slot `x` and read out during a designated time slot `y`. 1.ltoreq.x.ltoreq.N, 1.ltoreq.y.ltoreq.N, N being the number of slots per frame. The data of input-slot x and output-slot y would belong to the same frame if y.gtoreq.x. Otherwise, the data of slot y would be one frame old. If all the samples of a call are switched likewise, i.e., all in the same frame or all in the subsequent frame, then the connection is frame-consistent and, naturally, the samples can be switched in the proper order. Attempting to satisfy this condition with independent switching of the individual channels is subject to randomness and is likely to succeed only at very low occupancy and, even then, subject to certain restrictions. It should be remembered that in the switching process the selection of eligible free slots is usually subject to matching constraints which differ in nature according to the internal design of the switch. These eligibility conditions of free time slots are not altered by this invention. Call rearrangement (reswitching) of existing connections to accommodate a new arrival may be used to increase the traffic capacity (i.e., permissible mean occupancy at a specified grade of service) of the switching node. However, this is both impractical and hazardous.
Several solutions have been reported in the literature (an extensive survey is given in [1]). Generally, they fall under two categories: post-switching and en route delay equalization. Post-switching equalization does not result in traffic-capacity loss but it increases the switching delay since a deep buffer would be needed at the receiving end. It requires new hardware and complex software control. En route equalization, in turn, may be realized in two ways, by frame retention or clever call packing.
With particular regard to the frame retention technique, if the time switch is designed to store two consecutive frames, retaining an extra frame which would not be needed for single channel calls, then during an output time slot y belonging to frame f, the data of input time slot x belonging to frames (f, f-1), if y.gtoreq.x, or frames (f-1, f-2), if y&lt;x, would be available in the data memory. Thus, the data of time slot x of frame (f-1) is always present during frame f, regardless of the relative positions of x and y in the frame, and frame consistency is assured with an added round-trip delay of one frame per time switch.
The frame-retention technique does not reduce the traffic capacity. However, it doubles the switching delay and its implementation requires new switching nodes in which the time switches have deeper data memories (and wider addressing memories); it is therefore not suitable for multi-stage switching nodes.
The "call packing" techniques have been well studied for possible application in telephony switching to reduce matching loss in certain types of switching nodes. While, under the restriction of frame consistency, call-packing would offer significant advantages over first-encounter assignment of multi-channel calls, it is still wasteful of trunk-group capacity.
Packing is somewhat easier when the number of slots per frame is a power of two. A simple packing arrangement of a frame of N slots would be done with the help of a state vector of size N bits. Bit number i in the state vector stores the (busy/free) state of slot j, where j is the binary image of i; for example, in a 1024-slot frame (with the slots, numbered 0 to 1023), the index i:0011000001 (decimal 193) points to the state of frame slot j:1000001100 (decimal 524). The slot numbers need not be stored in the state vector. While such a scheme may be reasonably effective, it is still somewhat rigid, it results in traffic-capacity loss, and it increases the path assignment effort in the switch.